The present invention relates in general to a pressure relief subassembly for a chemical cutting tool used in oil and gas wells, and in particular to a selectively fired, pressure relief subassembly for a downhole chemical cutting tool.
Downhole, chemical cutting tools, often called chemical cutters, have been used to sever, or cut into two separate sections, steel tubular members within oil and gas wells. Typically, a chemical cutter is used when a lower section of a steel pipe string, such as a tubing string, a casing string, or a drill string, is stuck within a well, and it is desired to sever the lower section of the pipe string to allow retrieval of an upper portion of the pipe string from the well. A chemical cutter may be lowered within the stuck pipe string on a wireline to a position adjacent to the portion of the pipe string which is to be severed. Then, a flammable solid is ignited within the chemical cutter to force a liquid cutting chemical to flow over a chemical activator, and then outward of the chemical cutter through flow ports. The flow ports are arrayed for directing the activated cutting chemical to discharge in a pattern which extends circumferentially around the chemical cutter and into an annular-shaped section of the pipe string surrounding the chemical cutter. The cutting chemical and the activator are selected to provide high temperatures and pressures, such that the cutting chemical will cut through the adjacent section of the steel pipe string, severing the section into two halves. Activation of the cutting chemical and downhole well pressures expose the interior of the chemical cutter to high pressures, which should be released from being contained within the chemical cutter prior to the cutter being removed from within a well.
Prior art chemical cutting tools have interior chambers connected by flow passages within which high pressures may become trapped, causing safety concerns if high pressure fluids are unexpectedly released on the surface after chemical cutters are retrieved from within wells. Pressure bleed-off ports have been provided which are manually operated at the surface after chemical cutters are retrieved from wells, such as by providing a threaded plug which blocks a bleed-off port when the tool is downhole, and which is manually removed from blocking the bleed-off port after a chemical cutter is removed from a well. Bleed-off ports are often of a small diameter, and may become sealed by debris from the well. Unexpected discharges of trapped pressures and chemical cutting fluid at the surface after retrieval from wells have caused injuries to persons and damage to equipment.
A chemical cutter is provided having a pressure relief feature, such that after the chemical cutter is operated for dispensing a cutting chemical in a well to severe a tubular member, the pressure within the chemical cutter is equalized with the pressure which is exterior of the chemical cutter. An interior passage extends through a central portion of a tool housing of the chemical cutter. A propellant disposed in the interior passage, and is ignited for creating pressure to push a cutting chemical from within the tool housing and into the well. A first ignitor passage extends parallel to a longitudinal axis of the tool housing, and in fluid communication with the interior passage. A first ignitor is disposed in the first ignitor passage, such that ignition of the first ignitor ignites the propellant. A second ignitor passage extends in the tool housing, and has a first portion which extends parallel to the longitudinal axis, spaced apart from the first ignitor passage. An interior opening is provided in an end of the first portion of the ignitor passage which is adjacent to the interior passage. The second ignitor passage also has a second portion which extends transverse to the longitudinal axis of the tool housing, from an exterior of the tool housing to the first portion of the second ignitor passage. An exterior opening is provided in the outward end of the second portion of the second ignitor passage. A second ignitor is disposed in the second ignitor passage.
A first seal member is disposed in the interior opening, sealing the second ignitor passage from the interior passage of the tool housing. A second seal member disposed in the exterior opening, sealing the second ignitor passage from the exterior of the tool housing. The first and second seal members seal the second ignitor from the interior passage and from the exterior of the tool housing after the first ignitor is ignited and the propellant is combusted to dispense the cutting chemical from the cutting tool. Igniting the second ignitor pushes the first seal member from the interior opening and the second seal member from the exterior opening, such that the second ignitor passage is in fluid communication with the interior passage and the exterior of the tool housing. A control circuit is provided having two diodes connected in parallel, a first diode is configured for passing current of a first polarity to the first ignitor and a second diode is configured for passing current of a second polarity to the second ignitor.